The present invention relates to methods for setting the loop gain in a servo loop of a focusing servo device.
In an optical disc player for playing an information storage disc (referred to as disc, hereinafter) such as a video disc, a digital audio disc and the like, it is essential to provide a focusing servo device for focusing a light beam on the recording surface of the disc to form an information reading light spot. One type of focusing servo device uses the so-called astigmatism method, in which a focal error signal is generated on the basis of four signals from a four-piece detector. The detector is placed at the mid-point between the two points which are apart from each other in the light path of the beam and on each of which beams are converged by cylindrical lenses to form lines, for example, one in a horizontal direction and the other in a vertical direction.
However, in the focusing servo device, it is sometimes the case that a fluctuation in the laser light source power or a variation in the reflectivity of the disc causes the loop gain in the servo loop system to change with each playing of a disc. The fluctuation in the loop gain leads to a decline in tracking performance with respect to focusing errors, thereby preventing stabilized servo operation.